This invention relates generally to communication networks comprising interconnected computers and other devices. More particularly, the invention relates to local-area networks in which a number of communication stations or nodes are connected to a common communication bus. Networks of this general type for the interconnection of computers and other devices are becoming increasingly common. The principal objective in designing such a network is to provde a convenient way for the separate devices or nodes to transmit messages and information from one to another. Instead of having to provide costly interconnections between all possible pairs of the devices, a single communication bus is employed.
When one device has information to send to another, a message is transmitted onto the bus, and is read by the device to which the message is intended to be transmitted. One major task in designing a network of this type is to establish a scheme for resolving conflicts for the use of the bus. In this regard, there are two main types of network designs. One is what is commonly referred to as a contention scheme, in which conflicts for use of the bus are resolved at the time they arise, in accordance with a fixed set of priorities. If two nodes on the bus attempt to transmit at practically the same time, priority may be determined, for example, by the physical locations of the nodes, such that the node nearest one end of the bus is the first to transmit. One well known network system of the contention type is known by the name Ethernet, and is described in U.S. Pat. No. 4,063,220 to Metcalfe et al.
The other major type of network uses some form of time slot allocation scheme, wherein each of the nodes is assigned a time slot in which it may transmit on the bus. These are all basically time-division multiplexing schemes. In the simplest form of time slot allocation scheme, the node allocations are fixed, and the system may be very inefficient when not all of the nodes are equally busy. If the time slots are dynamically allocated, there is still a priority problem to be resolved. In the past, a system of fixed priorities based on some physical parameter has been the usual approach to resolving this difficulty.
Another important factor that has often dictated against the choice of a time slot allocation scheme is that some form of synchronization of the nodes is required, to make sure that each node can determine its proper time slot for transmission of data. This has necessitated the use of a master station at some location on the bus, to generate appropriate timing signals for use by the other nodes, which become, in effect, "slave" units. Reliance on a master station poses obvious reliability problems, since the integrity of the entire network then depends on a single master station.
It will be appreciated from the foregoing that there is a need for a local-area network that both avoids the use of fixed priorities for bus access, and avoids the use of a single master station for purposes of synchronization. The present invention fulfills this need.